Integrated Circuits (IC's) are the computer chips which run today's highly sophisticated electronic devices. Once an IC has been fabricated, it is placed inside an IC package, and the circuitry within the IC is electrically coupled to interconnects running through the package to leads on the external surface of the package. The IC is then permanently sealed within the package and is operated through the package walls from these external leads. It is the IC package which is responsible for not only protecting the IC from damage but also providing the IC with an environment in which the IC can operate at peak performance. However, current package technologies may not be suitable for supporting some of the more highly advanced IC's. Therefore, to exploit the full potential of these IC's, including next generation microprocessors and controllers, more highly advanced package design techniques must be employed.
There are many design constraints and operational parameters which must be accounted for when designing an IC package. For example, some IC's, particularly microprocessors and other devices which consume a large amount of power, generate a significant amount of heat while operating. If not adequately dissipated, this heat can degrade the performance of the IC by, for instance, slowing the operational speed of the device. In addition, if the heat generated by an IC is not readily removed, the IC can effectively destroy itself by causing its own circuitry to melt or become otherwise detrimentally altered. For example, junction spiking is a common mechanism by which the interconnects of an IC are shorted to the IC substrate in the presence of thermal activation, thereby destroying the device.
A popular method for dissipating the heat generated by an IC within a package is to thermally couple the bottom of the IC to the top of a large metal plate. This is typically done using a thermally conductive solder paste to attach the IC to the center of the metal plate. The bottom of the plate then forms a portion of an exterior wall of the package while the top of the plate is attached to the rim of the package, sealing the IC inside the package. When the IC is operated in this configuration, heat generated by the IC is conducted through the metal plate to the external surface of the package. This heat is then dissipated to the ambient environment.
Another consideration an IC package designer must account for is the amount of noise a particular IC can tolerate on the power and ground supply lines. Noise on a supply line can have the effect of varying the voltage on the line by a certain amount. If this amount is greater than a particular threshold value, the IC may interpret the varying voltage as an actual input signal and react accordingly, changing the state of its internal circuitry. This can cause IC data errors, or, in more severe situations, supply line noise can induce a phenomenon known as latchup, which destroys the IC altogether. As supply voltages continue to decrease and IC frequencies continue to increase, supply line noise problems become even more pernicious since, for example, the variation in voltage supply levels caused by the noise accounts for an increasingly greater proportion of the total supply voltage.
Unfortunately, methods for suppressing supply line noise are not compatible with the above described method of dissipating the heat generated by an IC. This is because IC supply line noise is suppressed by routing the power and ground lines to external leads on the surface of the package and attaching large, discrete capacitors to these leads. These large capacitors serve to filter high frequency noise from the supply lines, isolating the steady, direct current (dc) component of the supply for use by the IC. But the large metal plate used for heat dissipation occupies the area where these discrete capacitors would otherwise be placed. As a result, packages which incorporate a metal plate for heat dissipation must rely on the relatively small amount of capacitance which can be contained within the package for high frequency noise suppression. This amount of capacitance is generally too small to adequately filter much of the noise from the supply lines.
What is desired is an IC package which can provide the benefits associated with large capacitance noise filtering while remaining compatible with thermal dissipation techniques.